This invention relates in general to voltage control systems and in particular to a system for controlling the voltage between two objects where the voltage between the objects changes with the distance between them.
In many industrial processes it is desirable to control the amount of power supplied to be at a constant level. In welding, for example, it is frequently desirable to maintain the power supplied to welding torches at a constant level so that heat is supplied to the workpiece at a constant rate to achieve a smooth and even weld. In many welding processes, power sources supplying constant current are used to power the torches. In arc welding the power supplied by the welding torch to the workpiece is proportional to the product of the current and the voltage between the torch and the workpiece. The voltage between the torch and the workpiece may vary with the distance between them, so that when the torch is travelling over a workpiece of uneven height, the varying distance between the torchhead and the workpiece will cause the voltage between them to vary. While the current is kept constant, the voltage between the torch and the workpiece may still change. Thus, even though the power supply may supply a constant current flowing between the torch and workpiece through the arc, the amount of power delivered to the workpiece may still vary if the voltage between the torch and the workpiece varies. It is, therefore, desirable to provide a system to maintain the voltage between the torch and the workpiece at a constant level despite the varying distance between them.
Many conventional torch voltage control systems are analog systems. The reference or setpoint voltage is typically set using a potentiometer, where the desired voltage may, for example, by set by turning a knob. Other necessary parameters are set in a similar manner. Where more than one welding process is to be performed with the same reference voltage and other parameter setting, it is desirable to retain the same setting between two or more welding processes. The same setting is usually retained by simply leaving the knobs in the same positions once they have been set to the desired readings. Like other analog devices, potentiometers and knob settings are subject to drifts and small variations. Thus, while approximately the same settings will be retained for a period of time by leaving the knobs at the same positions, the drifts and small variations will cause the reference voltage and other parameters to wander away from the desired readings, causing inaccuracies in welding.
In conventional torch voltage control systems, an error signal is generated to indicate the deviation of the actual torch voltage from the desired reference voltage, and the distance between the torch and workpiece is corrected to reduce the deviation. In many systems, however, the error signal used for correcting the separation is merely proportional to the magnitude of the deviation of the torch voltage from the reference voltage. The error signal is not available until the torch voltage has already deviated from the reference voltage. Hence, the error voltage will always lag behind the deviation by a following error. Furthermore, such a correction scheme may not allow the error voltage to respond quickly to sudden changes in the separation between the torch and the workpiece. Thus, where the height of the workpiece changes suddenly causing a sudden change in the torch voltage, the torch voltage may differ significantly from the reference voltage.
Stepper motors have been used in conventional torch voltage control systems to move the torch away from or towards the workpiece in order to correct the torch voltage. Stepper motors each has a number of poles whose electromagnetic fields propell coils for making the stepwise movements. Where the number of steps per revolution of the motor corresponds to the number of poles, the motor is said to be operating in the full step mode. Where the number of steps per revolution is twice the number of poles, the motor is said to be operating in the half-step mode. Conventional torch voltage control systems use the stepper motors only in their full step modes. When the torch is close to the workpiece, the full size steps of the stepper motors may increase the chances for the torch to actually touch the workpiece, thereby extinguishing the arc and injecting impurities in the weld. It is therefore desirable to provide a voltage control system where the above described difficulties are alleviated.